Class Notes (808,131)
Canada (493,086)
Neuroscience (296)
NROC69H3 (37)
Lecture 2

Lecture 2 Notes.docx

15 Pages
Unlock Document

University of Toronto Scarborough
Michael Inzlicht

Lecture 2: Neurotransmission Chemical neurotransmission  Action potential down an axon to the nerve terminal  Opening of voltage-gated Ca 2+ channels in the presynaptic terminal  This allows the influx of Ca 2+  They float into the terminal  Ca 2+ leads to synaptic vesicles  Neurotransmitters bind to plasma membrane and release them into synaptic cleft  Post synaptic receptor  neurotransmitters gets diffused ** happens in less than 1 ms  Other neurotransmitters either get taken back up or they get degraded  Some of them move away in other areas ** volume transmission  They then diffuse across to bind to postsynaptic receptors either ionotropic or metabotropic Early studies on Neurotransmitter release  Fatt & Katz: discovered a spontaneous miniature end plate potential (MEPPs) of ~0.5 mV  They found this spontaneous MEPP at the neuromuscular junction of a frog during resting state  There was no stimulation but a 0.5 mV at resting phase  Presynaptic depolarization  stimulation of the motor neuron increased the frequency occurrence of MEPP but it did not change their amplitude  suggesting 1 Ach  binding of 1 Ach receptor  However, if MEPP is manipulated with either nAch Receptor blocker or acetylcholinesterase , Ach break down then the MEPP amplitude decreased  Ach release, ach receptor binding leads to MEPP  Suggesting MEPP must be reflecting the opening of many individual Ach receptors rather than one  since amplitude decreased  more MEPP more amplitude  When motor neuron is stimulated under condition that are unfavorable for neurotransmitter release (low Ca 2+), the EPP fluctuated in a stepwise manner  Smallest evoked EPPs were approx. the size of an MEPP & other EPPs were of sizes at integral multiples of MEPPs (doubles, triples, quadruples …) Quantal model of transmitter release  Quantal hypothesis, neurotransmitters are release from the presynaptic terminal in separate units or Quanta  Quantum is a discrete and Quanta 100s at once  Neurotransmitter release separated  you get the response on the EPP; example: 2 neurotransmitter release  a defined EPP  MEPP is a response to a spontaneous release of a single quantum  building block of EPP  in the absence of action potential, ach leaks into the neuromuscular junction causing very small depolarization in the postsynaptic  EPP is a response to some 100s of these quanta being released  these are the binding of neurotransmitters at the post synaptic terminals; these neurotransmitters binding to postsynaptic receptors will then cause depolarization in the postsynaptic terminals  EPP at threshold level  release of 100s MEPP quantum  Ca 2+ controls the release probability of the quanta  such that at normal levels, there would be sufficient Ca 2+ influx to release about a 100 quanta to generate an EPP of 40- 50 mV  However, at low Ca 2+ levels  there would be a failure of release as well as small EPPs As shown in the graph below: Anatomical evidence of vesicular/quantal release  At the neuron muscular junction, a single Ach channel opens and generates ~0.5 uv (micro volts)  Therefore, a quantal size of 0.5 mv MEPP  it should open 1000 Ach channels  Every Ach channel require 2 Ach molecules  Some of these molecules will be destroyed by esterase or will be lost in the synaptic cleft  Therefore, an estimate of 5000 Ach molecule is needed overall  A synaptic vesicle is estimated to contain about 1000-4000 molecules  A good correspondence between the number of Ach molecules in a quantum and the amount of Ach in a vesicle  If K+ channels are blocked then the Action potential is broadened and number of quanta can be increased  After freezing the membrane, they found holes in the active zones, which were vesicles  They found number of fused vesicles matched the number of quanta measured from the response  They also found pits outside of the cell and figured these were vesicles that can be recycled  The vesicles fuse fully with the membrane at the active zone, and that the membrane is then taken from a region of the presynaptic terminal away from the active zone and used to form new vesicles which are then filled with synaptic transmitter molecules known as full fusion model  Another evidence to know it’s a full fusion evidence, is knowing the incorporation of vesicle membrane into the cell membrane leads to an increase in the total cell surface area of the presynaptic membrane  given the capacitance measure is a function of the membrane surface area and the membrane capacitance Cm Cm is likely to remain constant however; increase in capacitance measure is likely to reflect the full fusion An example used to prove that statement was “whole cell patch clamp”  A small part of the membrane of an active vesicle fuses with and forms a pore through the region of the presynaptic terminal facing the synaptic cleft, the vesicle discharges the transmitter molecules into the synaptic cleft, it then vesicle then separates itself from the membrane and moves to the interior of the bouton to have its store transmitter molecules refilled. This rapid mechanism is called kiss and run model  Another evidence to know it’s a kiss and run model - a low level of signal, probably representing leakage of neurotransmitter through a small flickering fusion pore (100 ms) and leads a spike that results from the rapid dilation of the fusion pore and hence the release of the remaining contents of the granule Small flicker is known as foot and the full transmitter release is known as fusion pore; shown in the diagram below  Full fusion model evidence was found through capacitance and the whole cell patch clamp; kiss and run model evidence was found through amperometric current by using the carbon fibre electrode which monitors the amount of neurotransmitters released from the patch of membrane This shows both the evidence of full fusion and kisses and run both taking place Molecular evidence for vesicular (kiss and run) fusion  Vesicular fusion with the plasma membrane requires the coordinated activity of a number of core protein molecules located on the vesicle and cell membrane called Snare proteins  These proteins contain synaptobrevin  resides on the vesicle membrane (V-SNARE) , SNAP-25 and syntaxin both resides on the plasma membrane (t-SNARE) Steps of the SNARE proteins 1- Fusion requires the ‘docking’ of the vesicle with the cell membrane to form a loose trans- SNARE 2- This complex trans SNARE will then tighten trans-SNARE complex and ready to initiate the formation of a pore through the fused vesicle and cell membrane ‘priming’ 3- The resulting opening of the fusion pore is a Ca 2+ dependent process that leads to a change in the SNARE complex from a trans to cis 4- Once the formation changes, it allows the pore to open fully to allow the vesicle to release its content while it is in the cis formation 5- The vesicle then disengage from the cell membrane for another cycle of fusion ‘endocytosis’  A unique feature of the synaptic transmission is the tight coupling between the increase in intracellular Ca 2+  which is produced by the arrival of a nerve impulse and the initiation of the exocytotic synaptic vesicle fusion  The probability of vesicle fusion increases within less than 0.2 ms after Ca 2+ influx and returns to lower level at 1 ms  Synaptotagmin  Calcium sensor; it has two calcium binding sites and a phospholipid binding property  If this gene that codes for Synaptotagmin is knocked out then the neurotransmitter release is disrupted as shown below with the syt mutant  A readily releasable pool of vesicles located at or close to the active zone of the presynaptic membrane. Kiss and run vesicles that reform rapidly after electrical stimulation of neuron  A reserve pool of vesicle located away from the active zone of the presynaptic membrane and these vesicles reforms slowly from the infolding of the cell membrane  During intense electrical stimulation of neuron  the reserve pools are mobilized 10-15 s & migrate to the active zone Post-synaptic mechanism  Many of the neurotransmitters can cause more than 1 post-synaptic response  Same neurotransmitters can have different effects at different regions  Different neurotransmitter modify the same ionic currents while acting through its own separate receptor Post-synaptic excitation  Excitation increase the probability of action potential discharge by depolarization  achieved by opening of non-selective cation channels or closing of anion or K+ channels  Degree of excitation  depend on a) Amount and frequency of neurotransmitter release b) The life time of neurotransmitter release in the synaptic cleft c) The responsiveness d) Number and location of the post synaptic receptors e) Excitation state of the post-synaptic cell  Excitatory post synaptic currents EPSC or potential EPSP can be fast or slow  Fast neurotransmission is usually mediated by the opening of ‘ionotropic receptors’ while slow neurotransmission is mediated by the opening of ‘metabotropic receptors’ Glutamate: main excitatory neurotransmitter in the CNS  It mediates both fast and slow forms of EPSP (potentials) that act upon both ionotropic and metabotropic receptors  Can achieve maximum amount of current with 1mM Glutamate ** Metabotropic  do not directly bind to anion channels  G-proteins act indirectly with anion channels Ionotropic glutamate receptors  Receptors act like ion channels  There are three major types of ionotropic glutamate receptors a) AMPA b) NMDA c) Kainate receptors  Glutamate binding to these receptors triggers the direct opening of non-selective cation channels (Na+, K+ Ca 2+)  They are composed of 4 functionally distinct subunits and therefore heterotetramers AMPA receptors  Fast EPSP  4 different subunits  GluA1-4, GluR1-4 or GluR A-D  Symmetric ‘dimer of dimers’  GluR2 (2units) and either GluR1,GluR3,GluR4 (2 units)  Can exist in 2 splice variant called ‘flip or flop’  a result of a small difference in the amino acid sequence in the S2 extracellular domain  Flip is more dominant during early development and ge
More Less

Related notes for NROC69H3

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.